Spin—Spin Coupling in 1-NMR Spectroscopy

Abstract

So far we have been concerned with positions at which different protons resonate. These signals, however, do not necessarily occur as single peaks but may be split into doublets, triplets, quartets, or multiplets. Thus ethyl chloride (Figure 2.1) would afford a triplet for the methyl group and a quartet for the methylene protons. This splitting occurs because nonequivalent protons attached to the same carbon or to adjacent carbon atoms exhibit spin-spin coupling. The splitting of the signals does not affect the overall integrated intensities of the multiplets which still correspond to the number of underlying protons. The magnitude of splitting is expressed as the “coupling constant,” J, in cycles per seconds or Hz. It is independent of the strength of the applied magnetic field but depends on the molecular stereochemistry and diminishes with, among other factors, an increasing number of bonds between the coupled protons. Coupling constants may be either positive or negative in sign, though by just looking at an NMR spectrum it is not possible to determine the sign of the J value.